Method and system for automatic frequency control optimization

ABSTRACT

A method and apparatus for automatic frequency control in a receiver of a wireless device, the method determining a channel estimation for a received signal; calculating a signal to noise ratio for the channel estimation; applying a weighting factor determined based on the calculated signal to noise ratio for the channel estimation to the channel estimation to create a weighted channel estimation; and supplying the weighted channel estimation to a voltage controlled oscillator.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/394,764, filed Feb. 27, 2009, the entire contents of whichare incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to automatic frequency control and inparticular to automatic frequency control systems for mobilecommunications.

BACKGROUND

In order to accurately demodulate data embedded in radio frequencysignals, the received signal needs to be converted to a basebandfrequency. In order to do this, the frequency of the transmitter shouldbe matched at the receiver.

In practice, the radio frequency signal received by the receiver isdistorted from the signal that was transmitted by the transmitter basedon the channel conditions. In order to overcome this, estimations aremade of the channel in an attempt to derive a gain and frequency fromknown data contained in the signal.

In low signal to noise ratio conditions errors in the estimatedfrequency can lead to undesirable fluctuations in a receiver's frequencycontrol loop.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be better understood with reference to thedrawings in which:

FIG. 1 is a block diagram of a conventional frequency correction system;

FIG. 2 is a block diagram of a frequency correction system having aweighting function based on channel conditions;

FIG. 3 is a block diagram of a process for determining a weightingfunction;

FIG. 4 is a flow diagram showing a method for the frequency controlsystem of FIG. 2; and

FIG. 5 is a block diagram of an exemplary mobile device that may be usedwith the method and system of the present disclosure.

DETAILED DESCRIPTION

The present disclosure provides a method for automatic frequency controlin a receiver of a wireless device comprising: determining a channelestimation for a received signal; calculating a signal to noise ratiofor the channel estimation; applying a weighting factor determined basedon the calculated signal to noise ratio for the channel estimation tothe channel estimation to create a weighted channel estimation; andsupplying the weighted channel estimation to a voltage controlledoscillator.

The present disclosure further provides a communications subsystem in amobile device comprising: a channel estimation block, the channelestimation block receiving a signal from a down converter and providinga channel estimation for phase and frequency errors in the signal; aphase differential block to determine a phase offset for the signal; afrequency offset block to determine a frequency error; a weightingfunction block configured to determine a weighting function based on asignal to noise ratio for the channel estimation, the weighting functionblock providing a weighted channel estimation; a converter block toconvert the weighted channel estimation to volts; and a voltagecontrolled oscillator receiving the converted weighted channelestimation and providing an input to the down converter.

When a user equipment is in poor channel conditions, or morespecifically when a user equipment is in a low signal-to-noise ratiocondition, the use of conventional techniques to improve estimates forthe transmitter frequency can degrade, leading to undesirablefluctuations in frequency estimates.

Reference is now made to FIG. 1. FIG. 1 shows a conventional frequencycontrol system 100 where a receiver 110 receives a downlink signal. Aswill be appreciated, such downlink signal comes from the base station tothe user equipment and various coding of the signal is possible.Examples of such coding include the global system for mobilecommunications (GSM), code divisional multiple access (CDMA), universalmobile telecommunications system (UMTS), wideband code division multipleaccess (WCDMA), among others.

In the present disclosure, a UMTS system and terminology associate withUMTS is utilized. However, this is not meant to be limiting and thepresent disclosure could equally applied to various radio technologies.

In the system 100, the received signal or symbol is converted to abaseband signal at multiplier 120.

A feedback loop is established where a channel estimation at channelestimation block 130 occurs. As will be appreciated, channel estimationblock 130 attempts to derive signal shift from known elements to findchanges in both the amplitude and the phase of the received signal fromthe expected values. Thus, if a constant phase shift is occurring, thisis determined at the channel estimation block 130.

The estimated signal is then provided to a phase differential block 140,in which an average phase shift is sought. As will be appreciated, undercertain channel conditions, the channel causes a stable phase shift inthe signal and the phase differential block 140 looks for the averagephase shift. Furthermore, different signaling systems can have differentmethods of frequency estimation that does not require a phasedifferential block. For example, GSM does not require a phasedifferential of the channel estimates for frequency estimation.

The output from the phase differential block 140 is provided to afrequency offset block 150. Frequency offset block 150 transfers thephase differentials to a frequency. The frequency is then fed to aconverter, which converts the frequency to a voltage as seen by block160. The voltage from block 160 is provided to a voltage controlledoscillator 170 which provides an oscillation that is input to multiplier120.

As seen from FIG. 1, the conventional frequency control system 100therefore estimates phase and amplitude adjustments to symbols. Thechannel estimate factors from block 130 are then used to extract theaverage phase difference over a set of received symbols at block 140,from which a frequency offset between the received signal and thecurrent oscillator setting is derived at block 150. This frequencyoffset is converted to volts from hertz at block 160 and applied to avoltage controlled oscillator 170 to adapt the down converter. In otherwords, the down converter requires multiplication by the centralfrequency plus the frequency offset derived in block 150.

The conventional frequency control system 100 therefore does not takeinto account the reliability of channel estimates from the channelestimation block 130. The channel estimates derived at channelestimation block 130 degrade in terms of their reliability as the signalto noise ratio is reduced. To introduce a factor of reliability, thepresent disclosure provides for a weighting function that is applied toeach frequency offset based on the quality of channel estimates fromwhich the frequency offset is derived.

In the present disclosure, the weighting function is a systemconfigurable function with parameters that are adjusted to theapplication, allowing the present disclosure to be used for variouspurposes.

In particular, the weighting function is a function of the signal tonoise ratio of the channel estimates. A low signal to noise ratio in amultipath or noisy environment can lead to the receiver to fail toconverge to the true frequency of the signal.

Reference is now made to FIG. 2.

The frequency control system 200 of FIG. 2 provides a down converter 220to down convert a received signal from receiver 210. The down converter220 further has a feedback function which has an input based on channelestimations, as in FIG. 1.

In particular, a channel estimation block 130, phase differential block140, frequency offset block 150, converter block 160 and voltagecontrolled oscillator 170 are provided and provide similar functionalityto those blocks in FIG. 1.

A weighting function block 230 is however added. The weighting functionblock 230, in one embodiment, provides a indication of the reliabilityof the channel estimation derived in block 130, thereby providing moreaccurate feedback to voltage controlled oscillator 170.

As will be appreciated, when a high signal-to-noise ratio exists, thequality of the received signal is high, providing for more accuratechannel estimation at channel estimation block 130. When thesignal-to-noise ratio is however low, the estimates derived by channelestimation block 130 can be corrupt and therefore need to be weightedwith appropriate reliability.

In one embodiment, a linear weighting function is possible. Oneexemplary weighting function is as follows:

$\begin{matrix}{w_{n} = {{f\left( {SNR}_{{Ch\_ Est},n} \right)} = \left\{ \begin{matrix}{1,} & {{{for}\mspace{14mu}{SNR}_{{Ch\_ Est},n}} \geq {SNR}_{\max}} \\{{\left( {{SNR}_{{Ch\_ Est},n} - {SNR}_{\min}} \right)/\left( {{SNR}_{\max -}{SNR}_{\min}} \right)},} & {{{for}\mspace{14mu}{SNR}_{\max}} > {SNR}_{{Ch\_ Est},n} > {SNR}_{\min}} \\{0,} & {{{for}\mspace{14mu}{SNR}_{{Ch\_ Est},n}} \leq {SNR}_{\min}}\end{matrix} \right.}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

From the above, the weight w for a particular frequency offset estimateis a function of the signal-to-noise ratio of the received signal fromwhich the frequency error was estimated. The function is linear and ifthe signal-to-noise ratio for the channel estimate is greater then amaximum predefined signal-to-noise ratio then the channel is consideredreliable and the weighting function is set to one. In one embodiment themaximum signal to noise ratio is the minimum signal to noise ratio plusa predetermined offset value.

Conversely, if the signal-to-noise ratio for the channel estimate isbetween the minimum signal-to-noise ratio and the maximumsignal-to-noise ratio, then a function can be derived. In the example ofFormula 1 above, the weighting function is the signal-to-noise ratio forthe channel estimate minus the minimum signal-to-noise ratio, alldivided by the difference between the maximum signal-to-noise ratio andthe minimum signal-to-noise ratio.

Finally, if the signal-to-noise ratio of the channel estimate is below aminimum signal-to-noise ratio then the weighting function can designatea zero for the sample or symbol.

The above is summarized with regard to FIG. 4. In particular, FIG. 4illustrates a flow diagram for the steps illustrated by the blockdiagram of FIG. 2.

The process in FIG. 4 starts at block 400 and proceeds to block 410 inwhich a channel estimation is determined for a received signal.

The process then proceeds to block 420 in which a signal to noise ratiois calculated for the channel estimation.

A weighting function is then applied in block 430 to the calculatedsignal to noise ratio to create a weighted channel estimation.

The process then proceeds to block 440 in which the weighted channelestimation is supplied to a voltage controlled oscillator which thenprovides the feedback as illustrated in FIG. 2.

The process then proceeds to block 450 and ends.

A method for implementing Formula 1 on a mobile device is illustratedwith reference to FIG. 3. In FIG. 3 a precondition is that a channelestimate with an estimated signal to noise ratio is received at block300.

The process then proceeds to block 310 in which a check is made todetermine whether the estimated signal to noise ratio is below a minimumsignal to noise ratio threshold. The minimum signal to noise ratiothreshold is represented as SNR_(min) in Formula 1 above.

If yes, the process proceeds to block 315 in which the weightingfunction is set to zero. As will be appreciated, this indicates tosubsequent processing elements that the channel estimate should beignored for frequency correction purposes.

Conversely, if it is found in block 310 that the estimated signal tonoise ratio is greater than the minimum signal to noise ratio threshold,the process proceeds to block 320. In block 320 a check is made todetermine whether the estimated signal to noise ratio is greater thanthe maximum signal to noise ratio.

If the estimated signal to noise ratio is greater than the maximumsignal to noise, then the estimate is determined to be reliable and theprocess proceeds to block 325 in which the weighting function is set toone.

If it is determined in block 320 that the estimated signal to noiseratio is not greater than the maximum signal to noise ratio, thecombination of blocks 310 and 320 indicate that the estimated signal tonoise ratio is between the minimum signal to noise ratio and the maximumsignal to noise ratio. In this case the process proceeds to block 330 inwhich the linear function as described in Formula 1 above is applied tothe weighting function. Specifically, utilizing Formula 1 above, theminimum signal to noise ratio is subtracted from the estimated signal tonoise ratio, and the result is divided by the difference between themaximum signal to noise ratio and minimum signal to noise ratio.

From blocks 315, 325 or 330 the process proceeds to block 340 and ends.

The weighting function, as illustrated by Formula 1 and FIG. 3 above, ismerely meant to be exemplary of a type of weighting function that can beadded to an automatic frequency control system. In alternateembodiments, other linear or non-linear functions could be utilized toweight the channel estimate.

The above therefore takes into account the minimum operating range of achannel estimation unit, which is the source of the derived frequencyoffsets or corrections. In this way, any deviation introduced by thechannel estimation unit when below an operating signal-to-noise ratioimplies a deviation to the derived frequency correction and could leadto loss of signal on a wireless handheld or loss of performance orreception in areas where the signal-to-noise ratio is at or below themaximum signal-to-noise ratio as described above.

As will be appreciated, the signal-to-noise ratio is provided above.However, in other technologies, the Energy per Chip (Eclo) is utilizedinstead of signal-to-noise ratio, but similar functionality can beprovided utilizing a weight function 230. Other measures of channelquality such as, but not limited to, bit error rates equally be used.All of the measures of channel quality are generally referred to hereinas “Signal to Noise Ratio” or “SNR”, and the present disclosure is notlimited to a particular measure of channel quality.

The use of the weighting function improves the performance of the mobilereceiver in areas where the signal-to-noise ratio is low, such as cellboundary regions, particularly during handover from third generation(3G) cells to second generation cells, among others.

The present system and methods could be utilized with a variety ofmobile devices. One exemplary mobile device is described below withreference to FIG. 5. This is not meant to be limiting, but is providedfor illustrative purposes.

FIG. 5 is a block diagram illustrating a mobile device capable of beingused with preferred embodiments of the apparatus and method of thepresent application. Mobile device 500 is preferably a two-way wirelesscommunication device having at least voice and data communicationcapabilities. Mobile device 500 preferably has the capability tocommunicate with other computer systems on the Internet. Depending onthe exact functionality provided, the wireless device may be referred toas a data messaging device, a two-way pager, a wireless e-mail device, acellular telephone with data messaging capabilities, a wireless Internetappliance, or a data communication device, as examples.

Where mobile device 500 is enabled for two-way communication, it willincorporate a communication subsystem 511, including both a receiver 512and a transmitter 514, as well as associated components such as one ormore, preferably embedded or internal, antenna elements 516 and 518,local oscillators (LOs) 513, and a processing module such as a digitalsignal processor (DSP) 520. As will be apparent to those skilled in thefield of communications, the particular design of the communicationsubsystem 511 will be dependent upon the communication network in whichthe device is intended to operate.

Network access requirements will also vary depending upon the type ofnetwork 519. In some CDMA networks network access is associated with asubscriber or user of mobile device 500. A CDMA mobile device mayrequire a removable user identity module (RUIM) or a subscriber identitymodule (SIM) card in order to operate on a CDMA network. The SIM/RUIMinterface 544 is normally similar to a card-slot into which a SIM/RUIMcard can be inserted and ejected like a diskette or PCMCIA card. TheSIM/RUIM card can have approximately 64K of memory and hold many keyconfiguration 551, and other information 553 such as identification, andsubscriber related information.

When required network registration or activation procedures have beencompleted, mobile device 500 may send and receive communication signalsover the network 519. As illustrated in FIG. 5, network 519 can consistof multiple base devices communicating with the mobile device. Forexample, in a hybrid CDMA 1×EVDO system, a CDMA base device and an EVDObase device communicate with the mobile device and the mobile device isconnected to both simultaneously. The EVDO and CDMA 1× base stations usedifferent paging slots to communicate with the mobile device.

Signals received by antenna 516 through communication network 519 areinput to receiver 512, which may perform such common receiver functionsas signal amplification, frequency down conversion, filtering, channelselection and the like, and in the example system shown in FIG. 5,analog to digital (A/D) conversion. A/D conversion of a received signalallows more complex communication functions such as demodulation anddecoding to be performed in the DSP 520. The communication subsystemcould include the feedback loop of FIG. 2 for demodulation.

In a similar manner, signals to be transmitted are processed, includingmodulation and encoding for example, by DSP 520 and input to transmitter514 for digital to analog conversion, frequency up conversion,filtering, amplification and transmission over the communication network519 via antenna 518. DSP 520 not only processes communication signals,but also provides for receiver and transmitter control. For example, thegains applied to communication signals in receiver 512 and transmitter514 may be adaptively controlled through automatic gain controlalgorithms implemented in DSP 520.

Mobile device 500 preferably includes a microprocessor 538 whichcontrols the overall operation of the device. Communication functions,including at least data and voice communications, are performed throughcommunication subsystem 511. Microprocessor 538 also interacts withfurther device subsystems such as the display 522, flash memory 524,random access memory (RAM) 526, auxiliary input/output (I/O) subsystems528, serial port 530, one or more keyboards or keypads 532, speaker 534,microphone 536, other communication subsystem 540 such as a short-rangecommunications subsystem and any other device subsystems generallydesignated as 542. Serial port 530 could include a USB port or otherport known to those in the art.

Some of the subsystems shown in FIG. 5 perform communication-relatedfunctions, whereas other subsystems may provide “resident” or on-devicefunctions. Notably, some subsystems, such as keyboard 532 and display522, for example, may be used for both communication-related functions,such as entering a text message for transmission over a communicationnetwork, and device-resident functions such as a calculator or tasklist.

Operating system software used by the microprocessor 538 is preferablystored in a persistent store such as flash memory 524, which may insteadbe a read-only memory (ROM) or similar storage element (not shown).Those skilled in the art will appreciate that the operating system,specific device applications, or parts thereof, may be temporarilyloaded into a volatile memory such as RAM 526. Received communicationsignals may also be stored in RAM 526.

As shown, flash memory 524 can be segregated into different areas forboth computer programs 558 and program data storage 550, 552, 554 and556. These different storage types indicate that each program canallocate a portion of flash memory 524 for their own data storagerequirements. Microprocessor 538, in addition to its operating systemfunctions, preferably enables execution of software applications on themobile device. A predetermined set of applications that control basicoperations, including at least data and voice communication applicationsfor example, will normally be installed on mobile device 500 duringmanufacturing. Other applications could be installed subsequently ordynamically.

A preferred software application may be a personal information manager(PIM) application having the ability to organize and manage data itemsrelating to the user of the mobile device such as, but not limited to,e-mail, calendar events, voice mails, appointments, and task items.Naturally, one or more memory stores would be available on the mobiledevice to facilitate storage of PIM data items. Such PIM applicationwould preferably have the ability to send and receive data items, viathe wireless network 519. In a preferred embodiment, the PIM data itemsare seamlessly integrated, synchronized and updated, via the wirelessnetwork 519, with the mobile device user's corresponding data itemsstored or associated with a host computer system. Further applicationsmay also be loaded onto the mobile device 500 through the network 519,an auxiliary I/O subsystem 528, serial port 530, short-rangecommunications subsystem 540 or any other suitable subsystem 542, andinstalled by a user in the RAM 526 or preferably a non-volatile store(not shown) for execution by the microprocessor 538. Such flexibility inapplication installation increases the functionality of the device andmay provide enhanced on-device functions, communication-relatedfunctions, or both. For example, secure communication applications mayenable electronic commerce functions and other such financialtransactions to be performed using the mobile device 500.

In a data communication mode, a received signal such as a text messageor web page download will be processed by the communication subsystem511 and input to the microprocessor 538, which preferably furtherprocesses the received signal for output to the display 522, oralternatively to an auxiliary I/O device 528.

A user of mobile device 500 may also compose data items such as emailmessages for example, using the keyboard 532, which is preferably acomplete alphanumeric keyboard or telephone-type keypad, in conjunctionwith the display 522 and possibly an auxiliary I/O device 528. Suchcomposed items may then be transmitted over a communication networkthrough the communication subsystem 511.

For voice communications, overall operation of mobile device 500 issimilar, except that received signals would preferably be output to aspeaker 534 and signals for transmission would be generated by amicrophone 536. Alternative voice or audio I/O subsystems, such as avoice message recording subsystem, may also be implemented on mobiledevice 500. Although voice or audio signal output is preferablyaccomplished primarily through the speaker 534, display 522 may also beused to provide an indication of the identity of a calling party, theduration of a voice call, or other voice call related information forexample.

Serial port 530 in FIG. 5, would normally be implemented in a personaldigital assistant (PDA)-type mobile device for which synchronizationwith a user's desktop computer (not shown) may be desirable, but is anoptional device component. Such a port 530 would enable a user to setpreferences through an external device or software application and wouldextend the capabilities of mobile device 500 by providing forinformation or software downloads to mobile device 500 other thanthrough a wireless communication network. The alternate download pathmay for example be used to load an encryption key onto the devicethrough a direct and thus reliable and trusted connection to therebyenable secure device communication. As will be appreciated by thoseskilled in the art, serial port 530 can further be used to connect themobile device to a computer to act as a modem.

Other communications subsystems 540, such as a short-rangecommunications subsystem, is a further optional component which mayprovide for communication between mobile device 500 and differentsystems or devices, which need not necessarily be similar devices. Forexample, the subsystem 540 may include an infrared device and associatedcircuits and components or a Bluetooth™ communication module to providefor communication with similarly enabled systems and devices.

The embodiments described herein are examples of structures, systems ormethods having elements corresponding to elements of the techniques ofthis application. This written description may enable those skilled inthe art to make and use embodiments having alternative elements thatlikewise correspond to the elements of the techniques of thisapplication. The intended scope of the techniques of this applicationthus includes other structures, systems or methods that do not differfrom the techniques of this application as described herein, and furtherincludes other structures, systems or methods with insubstantialdifferences from the techniques of this application as described herein.

The invention claimed is:
 1. A method for automatic frequency control ina receiver of a wireless device comprising: determining a channelestimation for a received signal; applying a weighting factor determinedbased on a channel quality for the channel estimation to the channelestimation to create a weighted channel estimation; and utilizing theweighted channel estimation for frequency control.
 2. The method ofclaim 1, wherein the weighting factor is determined based on apredetermined minimum signal to noise ratio.
 3. The method of claim 2,wherein the weighting factor is zero if the channel quality for thechannel estimation is less than the predetermined minimum channelquality.
 4. The method of claim 2, wherein the weighting factor isfurther determined based on a predetermined maximum channel quality, themaximum channel quality being a predetermined offset value above theminimum channel quality.
 5. The method of claim 4, wherein the weightingfactor is one if the channel quality for the channel estimation isgreater than the maximum channel quality.
 6. The method of claim 4,wherein the weighting factor is a linearly varied value if the channelquality for the channel estimation is between the predetermined minimumand maximum.
 7. The method of claim 6, wherein the weighting factorequals the minimum channel quality subtracted from the channel qualityof the channel estimation, the value of Which is further divided by thedifference between the maximum channel quality and the minimum channelquality.
 8. The method claim 1, further comprising converting theweighted channel estimation to volts.
 9. The method of claim 8, whereinthe utilizing comprising supplying the voltage from the weighted channelestimation to a voltage controlled oscillator, and further comprisingapplying an output from the voltage controlled oscillator to a downconverter.
 10. A frequency correction system in a receiver comprising: adown converter having a first input for a received signal and a secondinput: a channel estimation block, the channel estimation blockreceiving a signal from the down converter and providing a channelestimation; a weighting function block configured to determine aweighting function based on a channel quality for the channelestimation, the weighting function block providing a weighted channelestimation; a converter block to convert the weighted channel estimationto volts; and a voltage controlled oscillator receiving the convertedweighted channel estimation and providing the second input.
 11. Thecommunications subsystem of claim 10, wherein the weighting function isdetermined based on a predetermined minimum channel quality.
 12. Thefrequency correction system of claim 11, wherein the weighting factor iszero if the channel quality for the channel estimation is less than thepredetermined minimum channel quality.
 13. The frequency correctionsystem of claim 11, wherein the weighting factor is further determinedbased on a maximum channel quality, the maximum channel quality being apredetermined offset value above the minimum channel quality.
 14. Thefrequency correction system of claim 13, wherein the weighting factor isone if the channel quality of the channel estimation is greater than themaximum channel quality.
 15. The frequency correction system of claim13, wherein the weighting factor is a linearly varied value if thechannel quality for the channel estimation is between the predeterminedminimum channel quality and the maximum channel quality.
 16. Thefrequency correction system of claim 15, wherein the weighting factorequals the minimum channel quality subtracted from the channel qualityof the channel estimation, the value of which is further divided by thedifference between the maximum channel quality and the minimum channelquality.